This invention relates to a method and apparatus for measuring the permeability of a core sample and, more particularly, to such a method and apparatus whereby the permeability of the core sample is measured during nonsteady state flow conditions.
The permeability of a sample of material taken from a subterranean formation is an important measurement in evaluating the quantity and accessibility of petroleum deposits, natural gas formations, and the like that might be present in that subterranean formation. Permeability relates the volumetric flow rate of such petroleum or natural gas to a given pressure differential.
The unit of measure of permeability is expressed as a "Darcy", and the permeability of many subterranean reservoirs is expressed in units of "milli-darcies" (md) or "micro-darcies" (.mu.d). As may be appreciated, it is far simpler to recover useful material, such as natural gas and other useful fluids, from subterranean formations having relatively higher permeability. However, new technology has encouraged serious consideration of fluid recovery from subterranean rocks having relatively "tight" or low permeability. Consequently, there is a need to measure the permeability of relatively "tight" core samples, wherein the expected permeability is in the micro-darcy range. However, conventional permeability-measuring techniques generally rely on the so-called "steady-state" method wherein a fluid, such as air, flows through a core sample under tests at steady-state conditions. Such steady-state techniques often are time-consuming and inefficient. In addition, if air is used to measure the permeability of the sample, the permeability which is determined is the permeability of that sample to air at a specific mean pore pressure, which may be substantially different from the permeability of that same sample to different gases at mean pressures where the mean free paths of the gas molecules are different, and may also be substantially different from the permeability to oil or other suitable liquid. Because of these errors, which are a function of the "testing" fluid and mean pressure that is used for the measurements, correction factors, known as Klinkenberg factors, are available by which the measured permeability can be "corrected" so as to reflect permeability of the sample to a gas at infinite mean pressure.
To reduce the time required to measure permeability of core samples well below the 45 minutes attending steady-state techniques, a nonsteady state test has been described by Stanley C. Jones in "A Rapid Accurate Unsteady-State Klinkenberg Permeameter", Journal S.P.E., October 1972. However, test runs made in accordance with the technique disclosed by Jones often require as much time as the steady-state technique, especially for samples having low permeability.
Another technique that has been proposed for improving the time required to measure low permeabilities is the so-called "pulse decay technique" described by Walls et al. in "Effects of Pressure and Partial Water Saturation on Gas Permeability in Tight Sands: Experimental Results", presented at the 55th Annual Fall Technical Conference and Exhibition of the Society of the Petroleum Engineers, September 1980. The drawback in the technique disclosed by Walls et al. appears to be the requirement for an undesirably long equilibrium time before a sample can be run.
Other investigations have been made for techniques which will provide rapid, accurate measurements of the permeability of core samples. As a result of such investigations, the method and apparatus disclosed herein were developed. In accordance with the present invention, a measure of the permeability of a core sample may be made in about one-tenth the time required by steady state methods.